I found an answer for myself:

1 Boot Image from USB
2 checkout mtd-layout: cat /dev/mtd
mine is looking like this:
Code:
mtd0: 00060000 00020000 "Boot_firmware"
mtd1: 00320000 00020000 "Kernel"
mtd2: 0001fffe 00020000 "Fake_ROOT"
mtd3: 0001fffe 00020000 "Fake_DEV"
mtd4: 0001fffe 00020000 "Fake_APP"
mtd5: 03c00000 00020000 "Real_ROOT"
so the target for the kernel is mtd1
3 flash_eraseall /dev/mtd1 // not sure if this is necessary
4 flashcp /boot/uImage /dev/mtd1 //copy kernel from currently running usb-image
5 modify bootargs to use kernel from flash and root-fs from usb-stick
mine look like:
Code:
bootdesc_1=speedusb3
bootcmd_1=run bootargs_1;bootm 00060000
bootargs_1=set bootargs console=ttyAS0,115200 root=/dev/sdb3 rw init=/bin/devinit coprocessor_mem=4m@0x40000000,4m@0x40400000 printk=1 stmmaceth=ethaddr:00:1e:b8:01:5a:70 rootdelay=6 usb-storage.usbwait123=5:5:4